The biologic role of receptors for C3, the third component of complement, is firmly established. However, these receptors have been described so far in operational terms. This reflects limitations in knowledge of C3 receptor structure and details of their function. The plan is to obtain functional, immunochemical and structural characterization for the three known C3 receptors. We isolated and characterized a fluid phase C3b dimer and determined its binding isotherm to the C3b receptor on various human cells. We plan to carry out a similar analysis to determine binding characteristics of other fluid phase C3 fragments (e.g. C3bi and C3d) to their respective receptors. We will also identify, isolate and characterize monoclonal antibodies to C3 receptors. These will be used as probes for the isolation and physicochemical characterization of C3 receptor molecules. Effects of modulating receptor-ligand interaction on C3-dependent functions will be analyzed.